As I understand it, the psychology of addiction is associated with the Pluto archetype- as it seems that addiction is just an intensified separating desire. Addiction itself implies an opportunity for evolution.

1. Can the word addiction be appropriately used in the context of other signatures? (ie Venus in Libra: addicted to co-dependency) Or would Pluto, Scorpio or the 8th house need to be involved in order to use that word?

2. When addiction exists, I see that the soul has become identified with the object of addiction (I NEED...) The need manifests itself on a conscious level (thus through the Moon/nodes, the ego). This makes me wonder if the archetype of addiction is actually a Pluto and moon/lunar nodes thing?

For starters, Pluto relates to compulsion/repetition, the root of which is unconscious memories and desires within the Soul. An addiction is repetitive, so certainly there can be a Pluto component. Something is compulsive because the reasons for the attraction to it are below the threshold of awareness - emanating from the Soul, thus repeated and repeated seeking resolution, without understanding where the compulsion is coming from in the 1st place.

Pisces/Neptune relates to delusion/escapism, and is definitely associated with addictions. If you take it deeper, the deepest craving in any of us is to return to Source, to experience Divinity, that which is the only thing that can truly fill all our desires and places of lack. Addictions are a counterfeit way, an attempt at a shortcut method, of fulfilling those desires - what we become addicted to makes us feel that very deep sense of pleasure and satisfaction, at least initially. The feelings themselves are real. The issue is we identify the source of what we are feeling as whatever it is that appears to be letting us feel that way, a drug, alcohol, or deep love of/from another person. That is a mis-identification, and that becomes the root of the addiction. Through the repetition complex (Pluto) we keep repeating the experience of taking the substance, or connecting with the person, hoping that this time we will once again feel the way it made us feel in the beginning. Somehow it never does. Yet we get in a loop and keep repeating the experience.

In terms of the nodes, we are conscious of the pulls of the nodes because those are part of our sense of self, self-image. That relates to the "familiar feels secure" principle. So we repeat old patterns and habits. Pluto's compulsions lie beneath that level of awareness, in the unconscious. Both are operative.

You are right about Libra. Being a highly Libran person I can vouch for that. The EA term is "making the partner the defacto God or Goddess". That is, we experience a taste of divinity THROUGH the partner. We become confused (Neptune) and think what we are feeling is coming from the partner, not realizing it is actually coming from within us. We then project (Libra) on the partner that they are the source of the good feelings we experience, when they are actually our own 12th house Return to Source yearnings. No human can ever consistently live up to these projections. As the interactions proceed we tend to start feeling let down by the partner's imperfections and tend to start criticizing the partner's faults as we start experiencing them more where they really are at, which is always less than the potential we saw within them. And yet, we also remember how wonderful we feel at those moments when we reach a momentary peak with that partner. All that becomes the root of codependence.

In relationships EA describes this as the natural Yod between the 12th, 2nd, and 7th houses. That brilliant material is covered in Pluto Vol 2.